Monolayer transition metal dichalcogenides (TMDs) are ideal materials for atomically thin, flexible optoelectronic and catalytic devices. However, their optoelectrical performance such as quantum yield and carrier mobility often shows below theoretical expectations due to the existence of defects. For monolayer TMD-based devices, finding a low-cost, time-efficient, and nondestructive technique to visualize the change of defect distribution in the space domain and the defect-induced change of the carrier’s lifetime is vital for optimizing their optoelectronic properties. Here, we propose a microscopic pump-probe technique to map the defect distribution of monolayer TMDs. It is found that there is a linear relationship between transient differential reflection intensity and defect density, suggesting that this technique not only realizes the visualization of the defect distribution but also achieves the quantitative estimation of defect density. Moreover, the carrier lifetime at each point can also be obtained by the technique. The technique used here provides a new route to characterize the defect of monolayer TMDs on the micro-zone, which will hopefully guide the fabrication of high-quality two-dimensional (2D) materials and the promotion of optoelectrical performance.

We demonstrate a low-cost hybrid integrated and compact 100 GBaud four-lane coarse wavelength division multiplexing (CWDM) receiver optical sub-assembly (ROSA) based on an arrayed waveguide grating de-multiplexer in the O band. To achieve the horizontal light coupling between the planar light-wave circuit (PLC) based arrayed waveguide grating de-multiplexer and photodetector array, a 42° polished facet is applied for total reflection. A flexible printed circuit with high-frequency coplanar waveguides is used for a power supply of trans-impedance amplifier and signal transmission. The fabricated CWDM ROSA module, whose size is 18mm×22mm×6mm, shows a 3 dB bandwidth of 21.2, 18.4, 19.6, and 19.3 GHz, respectively, in each lane. The overall symbol error rates are at a magnitude of 10 7 for 25 GBaud four-level pulse amplitude modulation (PAM-4) transmission with an average input optical power of 5 dBm.

In this study, a point-scattering approach to the plane-wave optical transmission of subwavelength metal nanoslit arrays with varying angles of rotation and that of subwavelength metal supercell arrays consisting of nanoslits capable of various angles of rotation is developed. It is demonstrated that the suggested theories show good agreement with the simulations and experiments. The results show that constructive and destructive interference at each nanoslit can respectively enhance and suppress the surface plasmon polariton (SPP) far-field radiation of a metasurface. The proposed theory can predict the quantity and resonant wavelength of SPPs and provide a design scheme for an SPP device.

Optical absorbers with dynamic tuning features are able to flexibly control the absorption performance, which offers a good platform for realizing optical switching, filtering, modulating, etc. Here, we propose a thermally tunable broadband absorber applying a patterned plasmonic metasurface with thermo-chromic vanadium dioxide (VO2) spacers. An actively tunable absorption bandwidth and peak resonant wavelength in the region from the near- to mid-infrared (NMIR) are simultaneously achieved with the insulating–metallic phase transition of VO2. Moreover, the scalable unit cell, which is composed of multi-width sub-cells, provides a new freedom to further manipulate (i.e., broaden or narrow) the absorption bandwidth while maintaining a high relative absorption bandwidth and efficient absorbance at the same time. For both transverse-electric and transverse-magnetic polarizations, the proposed nanostructure exhibits a high absorption over a wide angular range up to 60°. This method holds a promising potential for versatile utilizations in optical integrated devices, NMIR photodetection, thermal emitters, smart temperature control systems, and so forth.

We investigate optical and electrical behaviors of a graphene saturable absorber (SA) and mode-locking performance of a graphene-SA-based mode-locked Er fiber laser in gamma-ray radiation. When irradiated up to 4.8 kGy at ～100 Gy/hr dose rate, the overall nonlinear transmittance in transverse electric mode was increased, while maintaining modulation depth to >10%. The corresponding polarization-dependent loss was reduced at a 1.2-dB/kGy rate. In the electrical properties, the charge carrier mobility was reduced, and the Dirac voltage shift was increased to positive under gamma-ray radiation. The radiation-induced optical and electrical changes turned out to be almost recovered after a few days. In addition, we confirmed that the graphene-SA-based laser showed stable CW mode-locking operation while the inserted graphene SA was irradiated for 2-kGy at a 45-Gy/hr dose rate, which corresponds to >40 years of operation in low Earth orbit satellites. To the best of our knowledge, this is the first evaluation of graphene SAs and graphene-SA-based mode-locked lasers in gamma-ray radiation, and the measured results confirm the high potential of graphene SAs and graphene-SA-based lasers in various outer-space environments as well as other radiation environments, including particle accelerators and radiation-based medical instruments.

Controlled obtaining of orbital angular momentum (OAM) modes of light at high power over arbitrary orders has important implications for future classical and quantum systems. Appreciable optical amplification has recently been observed for low-order or specific-order OAM modes. However, large amplification of high-order OAM modes still remains challenging. Here we report on flat-gain amplification of arbitrary OAM modes via Brillouin interactions and demonstrate that the OAM modes with various orders can be efficiently and relatively uniformly amplified by imaging the wave source of OAM mode propagation in a nonlinear medium. Meanwhile, the propagation properties of beams carrying OAM with arbitrary modes are high-fidelity maintained. This work provides a practicable way to flatten the mode gain and represents a crucial necessity to realize OAM mode filters with controllable mode gain bandwidth.

An all-optically reconfigurable generation of optical vortices would be highly beneficial to the implementation of next-generation optical communication and advanced information processing. The previously demonstrated approaches based on the parametric nonlinear optical processes, however, have exhibited limited conversion efficiency due to the group velocity mismatch and nonlinear phase shifts, and require the cumbersome preparation of either the optical element or initial seed beam having a non-zero topological charge. Here, we propose and analyze a novel scheme for highly efficient all-optical generation and control of optical vortices based on the dynamic acoustic vortex grating created by forward stimulated intermodal Brillouin scattering in a subwavelength-hole photonic waveguide. The dual-frequency pump beams in two different hybrid optical modes drive an acoustic vortex mode, which transforms a signal in the fundamental optical mode into an optical vortex mode. This scheme not only eliminates the need for the initial preparation of an angular-momentum-carrying medium or an optical vortex seed but also guarantees high modal purity and nearly 100% conversion efficiency assisted by the energy-momentum conservation. We also investigate the feasibility and practicability of the subwavelength-hole waveguides by examining the intermodal conversion efficiency and robustness of guidance of the optical vortices, taking into account the impact of the Kerr-type nonlinear effects on the intermodal Brillouin interactions based on our rigorous full-vectorial analytical theory.

Questions hovering over the modulation of bandgap size and excitonic effect on nonlinear absorption in two-dimensional transition metal dichalcogenides (TMDCs) have restricted their application in micro/nano optical modulator, optical switching, and beam shaping devices. Here, degenerate two-photon absorption (TPA) in the near-infrared region was studied experimentally in mechanically exfoliated MoS2 from single layer to multilayer. The layer-dependent TPA coefficients were significantly modulated by the detuning of the excitonic dark state (2p). The shift of the quasiparticle bandgap and the decreasing of exciton binding energy with layers were deduced, combined with the non-hydrogen model of excitons in TMDCs and the scaling rule of semiconductors. Our work clearly demonstrates the layer modulation of nonlinear absorption in TMDCs and provides support for layer-dependent nonlinear optical devices, such as optical limiters and optical switches.

In this paper, a novel baseband macromodeling framework for linear passive photonic circuits is proposed, which is able to build accurate and compact models while taking into account the nonidealities, such as higher order dispersion and wavelength-dependent losses of the circuits. Compared to a previous modeling method based on the vector fitting algorithm, the proposed modeling approach introduces a novel complex vector fitting technique. It can generate a half-size state-space model for the same applications, thereby achieving a major improvement in efficiency of the time-domain simulations. The proposed modeling framework requires only measured or simulated scattering parameters as input, which are widely used to represent linear and passive systems. Three photonic circuits are studied to demonstrate the accuracy and efficiency of the proposed technique.

Mid-infrared (mid-IR) imaging and spectroscopic techniques have been rapidly evolving in recent years, primarily due to a multitude of applications within diverse fields such as biomedical imaging, chemical sensing, and food quality inspection. Mid-IR upconversion detection is a promising tool for exploiting some of these applications. In this paper, various characteristics of mid-IR upconversion imaging in the femtosecond regime are investigated using a 4f imaging setup. A fraction of the 100 fs, 80 MHz output from a Ti:sapphire laser is used to synchronously pump an optical parametric oscillator, generating 200 fs mid-IR pulses tunable across the 2.7–4.0 μm wavelength range. The signal-carrying mid-IR pulses are detected by upconversion with the remaining fraction of the original pump beam inside a bulk LiNbO3 crystal, generating an upconverted field in the visible/near-IR range, enabling silicon-based CCD detection. Using the same pump source for generation and detection ensures temporal overlap of pulses inside the nonlinear crystal used for upconversion, thus resulting in high conversion efficiency even in a single-pass configuration. A theory is developed to calculate relevant acceptance parameters, considering the large spectral bandwidths and the reduced interaction length due to group velocity mismatch, both associated with ultrashort pulses. Furthermore, the resolution of this ultrashort-pulsed upconversion imaging system is described. It is demonstrated that the increase in acceptance bandwidth leads to increased blurring in the upconverted images. The presented theory is consistent with experimental observations.

A unique all-fiber interferometric sensor was proposed and successfully demonstrated efficient low-refractive-index liquid sensing in the range from 1.33 to 1.37, which is compatible with those of bio-liquids. A special silica coreless optical fiber with an open V-groove was used as an optical sensing medium, which provided a high sensitivity for a minute liquid volume in the nanoliter scale. The V-groove fiber (VGF) was serially concatenated between two single-mode fibers (SMFs). The LP₀₁ mode guided along the input SMF excited the higher-order modes in the VGF to generate multimode interference, whose spectrum was transmitted through the output SMF. A single liquid droplet with volume of ～80 nanoliters wet the entire hydrophilic surface of the VGF, and the transmission spectra shifted corresponding to its refractive index in a very linear manner. The sensor also showed a negligible temperature cross-sensitivity in the range 25°C–75°C, which overlaps with the biological temperature window such that the sensitivity of 159.696 nm per refractive index unit (nm/RIU) remained independent of the temperature variation. Modal properties of VGF were thoroughly analyzed numerically, and detailed processes for the sensor fabrication and sensing experiments were reported.

Acousto-optic interactions, employed in the ultrafast laser regulation, possess remarkable advantages for fast tuning performance in a wide spectral range. Here, we propose an ultrafast fiber laser whose wideband tunability is provided by an acousto-optic structure fabricated with an etched single-mode fiber. Because of the laser polarization conversion induced by the coupling between the core and cladding vector modes in the etched fiber, a band-pass characteristic of the acousto-optic interaction is achieved to effectively regulate the inner-cavity gain range. Cooperating with a saturable absorber based on single-wall carbon nanotubes (SWCNTs) with polarization robustness, a soliton operating state is achieved in the tunable erbium-doped fiber laser. By controlling the acoustical wave frequency from 1.039 to 1.069 MHz, this soliton laser can be conveniently tuned in a wide spectral range from 1571.52 to 1539.26 nm. Meanwhile, the laser pulses have near-transform-limited durations stably maintaining less than 2 ps at different wavelength channels, owing to the broadband nonlinear absorption of SWCNTs.

Ultrashort pulsed Bessel beams with intrinsic nondiffractive character and potential strong excitation confinement down to 100 nm can show a series of advantages over Gaussian beams in fabricating efficient Bragg grating waveguides (BGWs). In this work, we focus on parameter management for the inscription of efficient BGWs using the point-by-point method employing Bessel beams. Due to their high aspect ratio, the resulting one-dimensional void-like structures can section the waveguides and interact efficiently with the optical modes. Effective first-order BGWs with low birefringence can then be fabricated in bulk fused silica. By controlling the size and the relative location of grating voids via the Bessel pulse energy and scan velocities, the resonant behaviors of BGWs can be well regulated. A high value of 34 dB for 8 mm length is achieved. A simple predictive model for BGWs is proposed for analyzing the influences of processing parameters on the performance of BGWs. The technique permits multiplexing several gratings in the same waveguide. Up to eight grating traces were straightforwardly inscribed into the waveguide in a parallel-serial combined mode, forming the multiplex BGWs. As an application, the multiplex BGW sensor with two resonant peaks is proposed and fabricated for improving the reliability of temperature detection.

Refractive index (RI) sensing helps to identify biomolecules and chemicals in the mid-infrared range for drug discovery, bioengineering, and environmental monitoring. In this paper, we numerically demonstrate an electrically tunable RI sensor with ultrahigh sensitivity using a three-layer graphene nanoribbon array separated by hexagonal boron nitride (hBN). Unlike the weak resonance in single-layer graphene nanoribbons, a much stronger plasmon resonance featuring a higher-quality factor can be excited in the graphene/hBN few-layer ribbon array. Simultaneously, the high purity of graphene on hBN results in an outstanding charge mobility above 4×104 cm2·V 1·s 1 at 300 K, which allows a larger modulation depth. The interaction between the locally enhanced field around graphene ribbons and its surrounding analyte leads to ultrahigh sensitivity (4.207 μm/RIU), with the figure of merit reaching approximately 58. Moreover, this ultrasensitive detector could selectively work in different wavebands by controlling gate voltages applied to graphene. These merits of ultrahigh sensitivity and electrical tunability are major advances compared to previous RI sensors, paving a way toward ultrasensitive detection using graphene/hBN few-layer devices.

Quantum information protocols often rely on tomographic techniques to determine the state of the system. A popular method of encoding information is on the different paths a photon may take, e.g., parallel waveguides in integrated optics. However, reconstruction of states encoded onto a large number of paths is often prohibitively resource intensive and requires complicated experimental setups. Addressing this, we present a simple method for determining the state of a photon in a superposition of d paths using a rotating one-dimensional optical Fourier transform. We establish the theory and experimentally demonstrate the technique by measuring a wide variety of six-dimensional density matrices. The average fidelity of these with the expected state is as high as 0.9852±0.0008. This performance is comparable to or exceeds established tomographic methods for other types of systems.

We demonstrate a squeezing experiment exploiting the association of integrated optics and telecom technology as key features for compact, stable, and practical continuous variable quantum optics. In our setup, squeezed light is generated by single-pass spontaneous parametric down conversion on a lithium niobate photonic circuit and detected by a homodyne detector whose interferometric part is directly integrated on the same platform. The remaining parts of the experiment are implemented using commercial plug-and-play devices based on guided-wave technologies. We measure, for a CW pump power of 40 mW, a squeezing level of 2.00±0.05 dB(anti-squeezing 2.80±0.05 dB), thus confirming the validity of our approach and opening the way toward miniaturized and easy-to-handle continuous variable-based quantum systems.

The impact of operation current on the degradation behavior of 310 nm UV LEDs is investigated over 1000 h of stress. It ranges from 50 to 300 mA and corresponds to current densities from 34 to 201 A/cm2. To separate the impact of current from that of temperature, the junction temperature is kept constant by adjusting the heat sink temperature. Higher current was found to strongly accelerate the optical power reduction during operation. A mathematical model for lifetime prediction is introduced. It indicates that lifetime is inversely proportional to the cube of the current density, suggesting the involvement of Auger recombination.

The low modulation bandwidth of deep-ultraviolet (UV) light sources is considered as the main reason limiting the data transmission rate of deep-UV communications. Here, we present high-bandwidth III-nitride micro-light-emitting diodes (μLEDs) emitting in the UV-C region and their applications in deep-UV communication systems. The fabricated UV-C μLEDs with 566μm2 emission area produce an optical power of 196 μW at the 3400A/cm2 current density. The measured 3 dB modulation bandwidth of these μLEDs initially increases linearly with the driving current density and then saturates as 438 MHz at a current density of 71A/cm2, which is limited by the cutoff frequency of the commercial avalanche photodiode used for the measurement. A deep-UV communication system is further demonstrated. By using the UV-C μLED, up to 800 Mbps and 1.1 Gbps data transmission rates at bit error ratio of 3.8×10 3 are achieved assuming on-off keying and orthogonal frequency-division multiplexing modulation schemes, respectively.